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4. Linking pore network structure derived by microfocus X-ray CT to mass transport parameters in differently compacted loamy soils

Author

Baniya, Arjun, Kawamoto, Ken, Hamamoto, Shoichiro, Sakaki, Toshihiro, Saito, Takeshi, Muller, Karin, Moldrup, Per, and Komatsu, Toshiko

Subjects
Soil research, Soil structure -- Observations, Loams -- Observations, Soil permeability -- Observations, Permeability, Water wells, Hydrogeology, Local transit, Water, Seepage (Hydrology), CAT scans, Porosity, Tomography, Soils, Agricultural industry, Earth sciences
Abstract

Mass transport in soil occurs through the soil pore network, which is highly influenced by pore structural parameters such as pore-size distribution, porosity, pore tortuosity, and coordination number. In this study, we visualised the networks of meso- and macro-pores (typical pore radius r [greater than or equal to] 10 [micro]m) using micro-focus X-ray computed tomography (MFXCT) and evaluated pore structural parameters of two loamy soils from Japan and New Zealand packed at different degrees of compaction. The effect of compaction on pore structural parameters and relationships between pore structural parameters and measured mass transport parameters were examined. Results showed a clear influence of compaction on pore structural parameters, with the MFXCT-derived mean pore radii and pore tortuosities decreasing and the mean pore coordination number increasing with increasing dry bulk density. Especially, pores with r > 80 [micro]m became finer or were not well formed due to compaction. The MFXCT-derived pore structural parameters were not well correlated with the equivalent pore radii from measured water retention curves. However, volumetric surface areas and pore-network connectivity-tortuosity factors derived from MFXCT allowed a fair prediction of several important mass transport parameters such as saturated hydraulic conductivities, soil-gas diffusion coefficients, and soil-air permeabilities. Further studies are needed to link micro-pores with radii smaller than the X-ray CT resolution to meso- and macro-pores visualised by X-ray CT to improve the prediction of mass transport parameters in soil. Additional keywords: air permeability, hydraulic conductivity, microfocus X-ray computed tomography (MFXCT), pore network, soil gas diffusion coefficient., Introduction A better understanding of gas transport in soil is important to elucidate the emission of greenhouse gases (Kruse et al. 1996; Smith 2003; Kawamoto et al. 2006; Wickramarachchi et [...]

Published
2019
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6. マイクロフォーカスX 線CT による土壌間隙構造と物質移動係数の関連性に関する研究

Author

BANIYA, ARJUN

Abstract

Basically, the study was focused on visualization of soil pore structure and analysis as well as linking pore network structure derived from microfocus X-ray CT to mass transport in soils. In chapter 3, it was investigated the effects of compaction on pore structural parameters. We visualized soil-pore networks for two different loamy soils, a sandy loam from Japan and a silty clay loam from New Zealand, under different packing (Hand compaction, Standard and Modified Procter compaction) using a microfocus X-ray computed tomography (CT) system and analysed the pore-structural parameters. Mass transport parameters such as hydraulic conductivity, air permeability, gas diffusion coefficient was separately measured in the laboratory for these soils. Results showed that the mean pore diameter and pore tortuosity decreased, and mean pore coordination number increased with increasing densities. On the other hand, the MFXCT-derived pore structural parameters were not well correlated to the equivalent pore radii from water retention curves. However, volumetric surface areas and pore-network connectivity-tortuosity factors derived from MFXCT allowed a fair prediction of several important mass transport parameters such as saturated hydraulic conductivities, soil-gas diffusion coefficients, and soil-air permeabilities.In chapter 4, it was investigated the effects of moisture content on pore structural parameters. Undisturbed and repacked soil macropore networks were visualized for different soils: loam, silty clay loam and volcanic ash soils (Andosols) with different moisture contents using MFXCT system. Soil samples were prepared by different moisture content by saturated and subsequently drained to the desired soil-water potential using a hanging water column, pressure chamber and air-dry conditions. Mean pore tortuosity and pore coordination number decreased with increasing soil air filled porosity (decreasing moisture content). On the other hand, indirectly estimated pore connectivity tortuosity factor well represents the tortuosity of soil macropore from MFXCT. For both undisturbed and disturbed volcanic ash samples, the measured pore tortuosity from MFXCT (TMFXCT) and estimated pore tortuosity from gas transport (TG) became well corelated. This correlation indicated that indirectly estimated tortuosity factor well represents the tortuosity of soil macropore network. However, high variations were observed between pore diameter from MFXCT (dMFXCT) and pore diameter from gas transport (dG), indicating that the indirectly estimated equivalent pore diameter from soil gas transport parameters does not correspond to the mean pore diameter of soil macropore., ACKNOWLEDGEMENTS .................................................................................................... iList of Figures .................................................................................................... iiLists of Tables ..................................................................................................... vCHAPTER 1- GENERAL INTRODUCTION .................................................................................... 11.1 Background ...................................................................................................... 11.2 Objective of the study .......................................................................................... 41.3 Outline of Dissertation ......................................................................................... 41.4 References ...................................................................................................... 6CHAPTER 2- REVIEW OF APPLICATION OF MICROFOCUS X-RAY CT TECHNIQUE TO CHARACTERIZED SOIL PORE NETWORK STRUCTURE .... 10Abstract ........................................................................................................... 102.1 Introduction ................................................................................................... 102.2 Overview of microfocus X-ray computed tomography .......................................... 112.3 Application of X-ray CT for the characterization of pore structural parameters ................................. 122.4 Conclusion ..................................................................................................... 152.5 References ..................................................................................................... 16CHAPTER 3 - LINKING PORE NETWORK STRUCTURE DERIVED BY MICROFOCUS X-RAY CT TO MASS TRANSPORT PARAMETERS IN DIFFERENT COMPACTED SOILS .................................................................................................... 20Abstract ........................................................................................................... 203.1 Introduction ................................................................................................... 213.2 Materials and Methods .......................................................................................... 233.2.1 Materials .................................................................................................... 233.2.2 MFXCT measurements and analysis .............................................................................. 263.2.3 Measurements of mass transport parameters .................................................................... 303.3 Results and Discussion ......................................................................................... 343.3.1 Effects of compaction on pore structural parameters by MFXCT analysis ........................................ 343.3.2 Prediction of mass transport parameters by MFXCT driven pore structural parameters ........................... 413.4 Conclusion ..................................................................................................... 453.5 References ..................................................................................................... 46CHAPTER 4 - LINKING PORE NETWORK STRUCTURE DERIVED BY MICROFOCUS X-RAY CT TO MASS TRANSPORT PARAMETERS IN VARIABLE MOISTURE CONTENT SOILS ............................................................................................. 50Abstract ........................................................................................................... 504.1 Introduction ................................................................................................... 514.2 Materials and Methods .......................................................................................... 524.2.1 Materials .................................................................................................... 524.2.2 MFXCT measurements and analysis .............................................................................. 554.2.3 Measurements of mass transport parameters .................................................................... 604.4 Results and Discussion ......................................................................................... 634.4.1 Effects of moisture content on pore structural parameters by MFXCT analysis .................................. 634.4.2 Comparison of pore structural parameters from MFXCT and gas transport parameters ............................. 664.5 Conclusion ..................................................................................................... 714.6 References ..................................................................................................... 71CHAPTER 5- SUMMARY, CONCLUSION AND PERSPECTIVES .................................................................... 755.1 Summary ........................................................................................................ 755.2 Conclusion and Perspective ..................................................................................... 76APPENDIX A1 ........................................................................................................ 80, 指導教員 : 川本健

Published
2020

8. CHARACTERIZING PORE-STRUCTURAL PARAMETERS OF VOLCANIC ASH SOIL: COMPARISON BETWEEN NONDESTRUCTIVE AND INDIRECT METHODS.

Author

BANIYA, Arjun, Akihiro MATSUNO, and Ken KAWAMOTO

Subjects
VOLCANIC soils, ANDOSOLS, COMPUTED tomography, PORE size distribution, DIFFUSION, POROUS materials, SOIL air
Abstract

Mass transport within porous media is governed by their pore networks, which is highly influenced by pore structure parameters such as pore size distribution, porosity, pore tortuosity and pore coordination number. Micro-focus X-ray computed tomography (MFXCT) has emerged as a powerful nondestructive tool for the direct visualization and better understand soil pore geometry. In this study, soil macropore networks (typically, pore diameter ≥ 30 μm) were visualized and gas transport parameters were measured for volcanic ash soils taken from Nishi Tokyo City, Japan. Especially, the study aimed to identify the effect of moisture content on pore structural parameters based on MFXCT analysis and compare the MFXCT derived parameters to indirectly-estimated parameters from soil gas diffusion coefficient and air permeability such as pore tortuosity-connectivity factor (XG) and equivalent pore diameter (dG) for gas flow. Both undisturbed and repacked samples were used for characterizing soil pore networks. In MFXCT analysis, the pore structural parameters such as effective pore diameter (dMFXCT), pore tortuosity (TMFXCT), and coordination number (CMFXCT) were measured. Results showed the moisture content affected clearly pore structural parameters, TMFXCT and CMFXCT decreased with increasing soil air filled porosity, however, measured dMFXCT were independent on the moisture content. For both undisturbed and disturbed samples, the measured TMFXCT and XG became close and plotted in a narrow range between 1:1.5 and 1.5:1. On the other hand, high variations were observed between dMFXCT and dG, indicating that the indirectly estimated equivalent pore diameter from soil gas transport parameters does not correspond to the mean diameter of soil macropore. [ABSTRACT FROM AUTHOR]

Published
2020
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